Magnetic tape having characterized magnetic layer

ABSTRACT

A magnetic tape has an Ra measured regarding the surface of the magnetic layer of less than or equal to 1.8 nm, and a logarithmic decrement acquired by a pendulum viscoelasticity test performed regarding the surface of the magnetic layer of less than or equal to 0.050. The magnetic layer includes fatty acid ester. The full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the magnetic layer before and after performing vacuum heating with respect to the magnetic tape is respectively greater than 0 nm and less than or equal to 7.0 nm, and the difference between the spacing measured by optical interferometry regarding the surface of the magnetic layer after performing the vacuum heating with respect to the magnetic tape and the spacing measured before performing the vacuum heating is greater than 0 nm and less than or equal to 8.0 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119 to Japanese PatentApplication No. 2017-029500 filed on Feb. 20, 2017. The aboveapplication is hereby expressly incorporated by reference, in itsentirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recordingmedia and disk-shaped magnetic recording media, and tape-shaped magneticrecording media, that is, magnetic tapes are mainly used for storagesuch as data back-up. The recording and reproducing of information tothe magnetic tape are normally performed by allowing the magnetic tapeto run in a drive and bringing the surface of the magnetic layer of themagnetic tape to come into contact with a magnetic head (hereinafter,also simply referred to as a “head”) to slide thereon.

In the field of magnetic recording, the improvement of electromagneticconversion characteristics is constantly required. In regards to thispoint, JP2010-49731A, for example, discloses that a magnetic recordingmedium having excellent electromagnetic conversion characteristics isobtained by increasing surface smoothness of a magnetic layer (forexample, see paragraphs 0020 and 0178 of JP2010-49731A).

SUMMARY OF THE INVENTION

Increasing surface smoothness of a surface of a magnetic layer of amagnetic tape is an effective method for narrowing an interval (spacing)between a surface of a magnetic layer of a magnetic tape and a head toimprove electromagnetic conversion characteristics.

However, in such studies of the inventors, it was clear that a decreasein reproduction output is observed while repeating the running in themagnetic tape in which surface smoothness of the magnetic layer isincreased for improving electromagnetic conversion characteristics.

Therefore, an object of the invention is to provide a magnetic tapewhich shows excellent electromagnetic conversion characteristics and inwhich a decrease in reproduction output during repeated running isprevented.

According to one aspect of the invention, there is provided a magnetictape comprising: a non-magnetic support; and a magnetic layer includingferromagnetic powder and a binding agent on the non-magnetic support, inwhich a center line average surface roughness Ra measured regarding asurface of the magnetic layer is equal to or smaller than 1.8 nm, alogarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer is equal to orsmaller than 0.050, the magnetic layer includes fatty acid ester, a fullwidth at half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming vacuum heating with respect to the magnetic tape is greaterthan 0 nm and equal to or smaller than 7.0 nm, a full width at halfmaximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer after performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 7.0 nm, and a difference (S_(after)−S_(before))between a spacing S_(after) measured by optical interferometry regardingthe surface of the magnetic layer after performing the vacuum heatingwith respect to the magnetic tape and a spacing S_(before) measured byoptical interferometry regarding the surface of the magnetic layerbefore performing the vacuum heating with respect to the magnetic tapeis greater than 0 nm and equal to or smaller than 8.0 nm.

In the invention and the specification, the center line average surfaceroughness Ra measured regarding the surface of the magnetic layer of themagnetic tape (hereinafter, also simply referred to as a “magnetic layersurface roughness Ra”) is a value measured with an atomic forcemicroscope (AFM) in a region, of the surface of the magnetic layer,having an area of 40 μm×40 As an example of the measurement conditions,the following measurement conditions can be used. The magnetic layersurface roughness Ra shown in examples which will be described later isa value obtained by the measurement under the following measurementconditions. In addition, the “surface of the magnetic layer” of themagnetic tape is identical to the surface of the magnetic tape on themagnetic layer side.

The measurement is performed regarding the region of 40 μm×40 μm of thearea of the surface of the magnetic layer of the magnetic tape with anAFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tappingmode. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed(probe movement speed) is set as 40 μm/sec, and a resolution is set as512 pixel×512 pixel.

Hereinafter, the logarithmic decrement acquired by a pendulumviscoelasticity test performed regarding the surface of the magneticlayer is also simply referred to as “logarithmic decrement”.

In the invention and the specification, the logarithmic decrementdescribed above is a value acquired by the following method.

FIGS. 1 to 3 are explanatory diagrams of a measurement method of thelogarithmic decrement. Hereinafter, the measurement method of thelogarithmic decrement will be described with reference to the drawings.However, the aspect shown in the drawing is merely an example and theinvention is not limited thereto.

A measurement sample 100 is cut out from the magnetic tape which is ameasurement target. The cut-out measurement sample 100 is placed on asubstrate 103 so that a measurement surface (surface of the magneticlayer) faces upwards, in a sample stage 101 in a pendulumviscoelasticity tester, and the measurement sample is fixed by fixingtapes 105 in a state where obvious wrinkles which can be visuallyconfirmed are not generated.

A pendulum-attached columnar cylinder edge 104 (diameter of 4 mm) havingmass of 13 g is loaded on the measurement surface of the measurementsample 100 so that a long axis direction of the cylinder edge becomesparallel to a longitudinal direction of the measurement sample 100. Anexample of a state in which the pendulum-attached columnar cylinder edge104 is loaded on the measurement surface of the measurement sample 100as described above (state seen from the top) is shown in FIG. 1. In theaspect shown in FIG. 1, a holder and temperature sensor 102 is installedand a temperature of the surface of the substrate 103 can be monitored.However, this configuration is not essential. In the aspect shown inFIG. 1, the longitudinal direction of the measurement sample 100 is adirection shown with an arrow in the drawing, and is a longitudinaldirection of a magnetic tape from which the measurement sample is cutout. In the invention and the specification, the description regarding“parallel” includes a range of errors allowed in the technical field ofthe invention. For example, the range of errors means a range within±10° from an exact parallel state, and the error from the exact parallelstate is preferably within ±5° and more preferably within ±3°. Inaddition, as a pendulum 107 (see FIG. 2), a pendulum formed of amaterial having properties of being adsorbed to a magnet (for example,formed of metal or formed of an alloy) is used.

The temperature of the surface of the substrate 103 on which themeasurement sample 100 is placed is set to 80° C. by increasing thetemperature at a rate of temperature increase equal to or lower than 5°C./min (arbitrary rate of temperature increase may be set, as long as itis equal to or lower than 5° C./min), and the pendulum movement isstarted (induce initial vibration) by releasing adsorption between thependulum 107 and a magnet 106. An example of a state of the pendulum 107which performs the pendulum movement (state seen from the side) is shownin FIG. 2. In the aspect shown in FIG. 2, in the pendulumviscoelasticity tester, the pendulum movement is started by stopping(switching off) the electricity to the magnet (electromagnet) 106disposed on the lower side of the sample stage to release theadsorption, and the pendulum movement is stopped by restarting(switching on) the electricity to the electromagnet to cause thependulum 107 to be adsorbed to the magnetic 106. As shown in FIG. 2,during the pendulum movement, the pendulum 107 reciprocates theamplitude. From a result obtained by monitoring displacement of thependulum with a displacement sensor 108 while the pendulum reciprocatesthe amplitude, a displacement-time curve in which a vertical axisindicates the displacement and a horizontal axis indicates the elapsedtime is obtained. An example of the displacement-time curve is shown inFIG. 3. FIG. 3 schematically shows correspondence between the state ofthe pendulum 107 and the displacement-time curve. The stop (adsorption)and the pendulum movement are repeated at a regular measurementinterval, the logarithmic decrement A (no unit) is acquired from thefollowing Expression by using a displacement-time curve obtained in themeasurement interval after 10 minutes or longer (may be arbitrary time,as long as it is 10 minutes or longer) has elapsed, and this value isset as logarithmic decrement of the surface of the magnetic layer of themagnetic tape. The adsorption time of the first adsorption is set as 1second or longer (may be arbitrary time, as long as it is 1 second orlonger), and the interval between the adsorption stop and the adsorptionstart is set as 6 seconds or longer (may be arbitrary time, as long asit is 6 seconds or longer). The measurement interval is an interval ofthe time from the adsorption start and the next adsorption start. Inaddition, humidity of an environment in which the pendulum movement isperformed, may be arbitrary relative humidity, as long as the relativehumidity is 40% to 70%.

$\Delta = \frac{{\ln\left( \frac{A_{1}}{A_{2}} \right)} + {\ln\left( \frac{A_{2}}{A_{3}} \right)} + {\ldots\mspace{14mu}{\ln\left( \frac{A_{n}}{A_{n + 1}} \right)}}}{n}$

In the displacement-time curve, an interval between a point of theminimum displacement and a point of the next minimum displacement is setas a period of a wave. n indicates the number of waves included in thedisplacement-time curve in the measurement interval, and An indicatesthe minimum displacement and maximum displacement of the n-th wave. InFIG. 3, an interval between the minimum displacement of the n-th waveand the next minimum displacement is shown as Pn (for example, P₁regarding the first wave, P₂ regarding the second wave, and P₃ regardingthe third wave). In the calculation of the logarithmic decrement, adifference (in Expression A_(n+1), in the displacement-time curve shownin FIG. 3, A₄) between the minimum displacement and the maximumdisplacement appearing after the n-th wave is also used, but a partwhere the pendulum 107 stops (adsorption) after the maximum displacementis not used in the counting of the number of waves. In addition, a partwhere the pendulum 107 stops (adsorption) before the maximumdisplacement is not used in the counting of the number of waves, either.Accordingly, the number of waves is 3 (n=3) in the displacement-timecurve shown in FIG. 3.

Hereinafter, the full width at half maximum (FWHM) of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer before performing the vacuum heating with respect tothe magnetic tape is also referred to as “FWHM_(before)” and the fullwidth at half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape is alsoreferred to as “FWHM_(after)”. The difference between a spacingS_(after) measured by optical interferometry regarding the surface ofthe magnetic layer after performing the vacuum heating with respect tothe magnetic tape and a spacing S_(before) measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming the vacuum heating with respect to the magnetic tape is alsoreferred to as a “difference (S_(after)−S_(before))”. The FWHM_(after)and the spacing S_(after) for acquiring the difference(S_(after)−S_(before)) are values acquired after performing the vacuumheating with respect to the magnetic tape. In the invention and thespecification, the “vacuum heating” of the magnetic tape is performed byholding the magnetic tape in an environment of a pressure of 200 Pa to0.01 MPa and at an atmosphere temperature of 70° C. to 90° C. for 24hours.

In the invention and the specification, the spacing measured by opticalinterferometry regarding the surface of the magnetic layer of themagnetic tape is a value measured by the following method.

In a state where the magnetic tape and a transparent plate-shaped member(for example, glass plate or the like) are overlapped with each other sothat the surface of the magnetic layer of the magnetic tape faces thetransparent plate-shaped member, a pressing member is pressed againstthe side of the magnetic tape opposite to the magnetic layer side at apressure of 5.05×10⁴ N/m (0.5 atm). In this state, the surface of themagnetic layer of the magnetic tape is irradiated with light through thetransparent plate-shaped member (irradiation region: 150,000 to 200,000μm²), and a spacing (distance) between the surface of the magnetic layerof the magnetic tape and the surface of the transparent plate-shapedmember on the magnetic tape side is acquired based on intensity (forexample, contrast of interference fringe image) of interference lightgenerated due to a difference in a light path between reflected lightfrom the surface of the magnetic layer of the magnetic tape andreflected light from the surface of the transparent plate-shaped memberon the magnetic tape side. The light emitted here is not particularlylimited. In a case where the emitted light is light having an emissionwavelength over a comparatively wide wavelength range as white lightincluding light having a plurality of wavelengths, a member having afunction of selectively cutting light having a specific wavelength or awavelength other than wavelengths in a specific wavelength range, suchas an interference filter, is disposed between the transparentplate-shaped member and a light reception unit which receives reflectedlight, and light at some wavelengths or in some wavelength ranges of thereflected light is selectively incident to the light reception unit. Ina case where the light emitted is light (so-called monochromatic light)having a single luminescence peak, the member described above may not beused. The wavelength of light incident to the light reception unit canbe set to be 500 to 700 nm, for example. However, the wavelength oflight incident to the light reception unit is not limited to be in therange described above. In addition, the transparent plate-shaped membermay be a member having transparency through which emitted light passes,to the extent that the magnetic tape is irradiated with light throughthis member and interference light is obtained.

The measurement described above can be performed by using a commerciallyavailable tape spacing analyzer (TSA) such as Tape Spacing Analyzermanufactured by MicroPhysics, Inc., for example. The spacing measurementof the examples was performed by using Tape Spacing Analyzermanufactured by Micro Physics, Inc.

In addition, the full width at half maximum of spacing distribution ofthe invention and the specification is a full width at half maximum(FWHM), in a case where the interference fringe image obtained by themeasurement of the spacing described above is divided into 300,000points, a spacing of each point (distance between the surface of themagnetic layer of the magnetic tape and the surface of the transparentplate-shaped member on the magnetic tape side) is acquired, this spacingis shown with a histogram, and this histogram is fit with Gaussiandistribution.

Further, the difference (S_(after)−S_(before)) is a value obtained bysubtracting a mode before the vacuum heating from a mode after thevacuum heating of the 300,000 points.

In one aspect, the logarithmic decrement is 0.010 to 0.050.

In one aspect, the center line average surface roughness Ra is 1.2 nm to1.8 nm.

In one aspect, the FWHM_(before) is 3.0 nm to 7.0 nm.

In one aspect, the FWHM_(after) is 3.0 nm to 7.0 nm.

In one aspect, the difference (S_(after)−S_(before)) is 2.0 nm to 8.0nm.

In one aspect, the magnetic tape further comprises a non-magnetic layerincluding non-magnetic powder and a binding agent between thenon-magnetic support and the magnetic layer.

In one aspect, the magnetic tape further comprises a back coating layerincluding non-magnetic powder and a binding agent on a surface side ofthe non-magnetic support opposite to a surface side provided with themagnetic layer.

In the magnetic tape according to one aspect of the invention, it ispossible to exhibit excellent electromagnetic conversion characteristicsand prevent a decrease in reproduction output during repeated running.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram of a measurement method of alogarithmic decrement.

FIG. 2 is an explanatory diagram of the measurement method of alogarithmic decrement.

FIG. 3 is an explanatory diagram of the measurement method of alogarithmic decrement.

FIG. 4 shows an example (step schematic view) of a specific aspect of amagnetic tape manufacturing step.

FIG. 5 is a schematic configuration diagram of a vibration impartingdevice used in examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention relates to a magnetic tape including: anon-magnetic support; and a magnetic layer including ferromagneticpowder and a binding agent on the non-magnetic support, in which acenter line average surface roughness Ra measured regarding the surfaceof the magnetic layer (magnetic layer surface roughness Ra) is equal toor smaller than 1.8 nm, a logarithmic decrement acquired by a pendulumviscoelasticity test performed regarding the surface of the magneticlayer is equal to or smaller than 0.050, the magnetic layer includesfatty acid ester, a full width at half maximum of spacing distributionmeasured by optical interferometry regarding the surface of the magneticlayer before performing vacuum heating with respect to the magnetic tape(FWHM_(before)) is greater than 0 nm and equal to or smaller than 7.0nm, a full width at half maximum of spacing distribution measured byoptical interferometry regarding the surface of the magnetic layer afterperforming the vacuum heating with respect to the magnetic tape(FWHM_(after)) is greater than 0 nm and equal to or smaller than 7.0 nm,and a difference (S_(after)−S_(ham)) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape and a spacing S_(before) measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 8.0 nm.

Hereinafter, the magnetic tape will be described more specifically.

Magnetic Layer Surface Roughness Ra

The center line average surface roughness Ra measured regarding thesurface of the magnetic layer of the magnetic tape is equal to orsmaller than 1.8 nm. Accordingly, the magnetic tape can exhibitexcellent electromagnetic conversion characteristics. From a viewpointof further improving the electromagnetic conversion characteristics, themagnetic layer surface roughness Ra is preferably equal to or smallerthan 1.7 nm and even more preferably equal to or smaller than 1.6 nm. Inaddition, the magnetic layer surface roughness Ra can be equal to orgreater than 1.0 nm or equal to or greater than 1.2 nm. However, from aviewpoint of improving the electromagnetic conversion characteristics,low magnetic layer surface roughness Ra is preferable, and thus, themagnetic layer surface roughness Ra may be lower than the exemplifiedlower limit.

The magnetic layer surface roughness Ra can be controlled by awell-known method. For example, the magnetic layer surface roughness Racan be changed in accordance with the size of various powders includedin the magnetic layer or manufacturing conditions of the magnetic tape.Thus, by adjusting one or more of these, it is possible to obtain amagnetic tape having the magnetic layer surface roughness Ra equal to orsmaller than 1.8 nm.

The inventors have found that, in the magnetic tape having the magneticlayer surface roughness Ra equal to or smaller than 1.8 nm, in a casewhere any measures are not prepared, the reproduction output isdecreased while repeating running Although the reason of a decrease inreproduction output is not clear, it is found that the decrease inreproduction output significantly occurs in a case of repeated runningof the magnetic tape at a high speed in an environment of a hightemperature and high humidity. The environment of a high temperature andhigh humidity here is, for example, an environment in which anatmosphere temperature is 30° C. to 45° C. and relative humidity isequal to or higher than 65% (for example, 65% to 90%). The running at ahigh speed is, for example, running of the magnetic tape at a runningspeed equal to or higher than 6.0 msec.

Therefore, as a result of intensive studies of the inventors, theinventors have newly found that it is possible to prevent a decrease inreproduction output during repeated running at a high speed in theenvironment of a high temperature and high humidity, by respectivelysetting the logarithmic decrement, the FWHM_(before), the FWHM_(after),and the difference (S_(after)−S_(before)) as described above, in themagnetic tape having the magnetic layer surface roughness Ra equal to orsmaller than 1.8 nm. Details of this point will be described later.

It is thought that the decrease in reproduction output occurs becausecomponents derived from the magnetic tape are attached to the head fromthe surface of the magnetic layer due to continuous sliding between thesurface of the magnetic layer and the head at the time of repeating therunning of the magnetic tape, and the attached components (hereinafter,referred to as “head attached materials”) exist between the surface ofthe magnetic layer and the head (so-called spacing loss). Thus, theinventors have made research for decreasing the amount of the componentsattached to the head from the surface of the magnetic layer. As aresult, the inventors have considered that the logarithmic decrement,the FWHM_(before), the FWHM_(after), and the difference(S_(after)−S_(before)) set as described above contribute to a decreasein amount of the components attached to the head from the surface of themagnetic layer. The logarithmic decrement, the FWHM_(before), theFWHM_(after), and the difference (S_(after)−S_(before)) will bedescribed later specifically. By doing so, the magnetic tape accordingto one aspect of the invention has been completed. However, the aboveand following descriptions include the surmise of the inventors. Theinvention is not limited to such a surmise.

Logarithmic Decrement

The logarithmic decrement acquired by a pendulum viscoelasticity testperformed regarding the surface of the magnetic layer of the magnetictape is equal to or smaller than 0.050. This can contribute toprevention of a decrease in reproduction output, in a case of therepeated running of the magnetic tape having the magnetic layer surfaceroughness Ra in the range described above. From a viewpoint of furtherpreventing a decrease in reproduction output, the logarithmic decrementis preferably equal to or smaller than 0.048, more preferably equal toor smaller than 0.045, even more preferably equal to or smaller than0.040, and still more preferably equal to or smaller than 0.035. Inaddition, the logarithmic decrement can be, for example, equal to orgreater than 0.010, equal to or greater than 0.012, or equal to orgreater than 0.015. From a viewpoint of preventing a decrease inreproduction output, the logarithmic decrement tends to be preferable,as it is low. Therefore, the logarithmic decrement may be lower than thelower limit exemplified above.

The inventors have considered regarding the logarithmic decrementdescribed above as follows. However, the below description is merely asurmise and the invention is not limited thereto.

It is possible to improve electromagnetic conversion characteristics byincreasing the surface smoothness of the surface of the magnetic layerof the magnetic tape. Meanwhile, it is thought that, in a case where thesurface smoothness is increased, a contact area (so-called real contactarea) between the surface of the magnetic layer and the head duringrepeated running increases. Accordingly, the inventors have surmisedthat components derived from the magnetic tape are easily attached tothe head from the surface of the magnetic layer and are attached andaccumulated on the head while repeating the running, thereby causingspacing loss which is a reason of a decrease in reproduction output.With respect to this, the inventors have thought that the componentsattached and accumulated on the head include pressure sensitive adhesivecomponents separated from the surface of the magnetic layer. Inaddition, the inventors have considered that the logarithmic decrementis a value which may be an index for the amount of the pressuresensitive adhesive components and the value equal to or smaller than0.050 means a decrease in amount of the pressure sensitive adhesivecomponents attached to the head from the surface of the magnetic layer.The details of the pressure sensitive adhesive components are not clear,but the inventors have surmised that the pressure sensitive adhesivecomponents may be derived from a resin used as a binding agent. Thespecific description is as follows. As a binding agent, various resinscan be used as will be described later in detail. The resin is a polymer(including a homopolymer or a copolymer) of two or more polymerizablecompounds and generally also includes a component having a molecularweight which is smaller than an average molecular weight (hereinafter,referred to as a “binding agent component having a low molecularweight”). The inventors have surmised that the binding agent componenthaving a low molecular weight which is separated from the surface of themagnetic layer during the running and attached and accumulated on thehead while repeating the running may cause the spacing loss which is areason of a decrease in reproduction output. The inventors have surmisedthat, the binding agent component having a low molecular weight may havepressure sensitive adhesive properties and the logarithmic decrementacquired by a pendulum viscoelasticity test may be an index for theamount of the component attached and accumulated on the head during therunning. In one aspect, the magnetic layer is formed by applying amagnetic layer forming composition including a curing agent in additionto ferromagnetic powder and a binding agent onto a non-magnetic supportdirectly or with another layer interposed therebetween, and performingcuring process. With the curing process here, it is possible to allow acuring reaction (crosslinking reaction) between the binding agent andthe curing agent. However, although the reason thereof is not clear, theinventors have considered that the binding agent component having a lowmolecular weight may have poor reactivity regarding the curing reaction.Accordingly, the inventors have surmised that the binding agentcomponent having a low molecular weight which hardly remains in themagnetic layer and is easily separated from the surface of the magneticlayer and attached to the head may be one of reasons for that thebinding agent component having a low molecular weight is attached andaccumulated on the head during the running.

A specific aspect of a method for adjusting the logarithmic decrementwill be described later.

Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) between the spacings measuredregarding the magnetic tape before and after performing the vacuumheating is greater than 0 nm and equal to or smaller than 8.0 nm.Regarding the difference (S_(after)−S_(before)), the inventors havethought that this value can be an index for a thickness of a liquid filmformed on the surface of the magnetic layer with fatty acid esterincluded in the magnetic layer. Specific description is as follows.

A portion (projection) which mainly comes into contact (so-called realcontact) with the head in a case where the magnetic tape and the headslide on each other, and a portion (hereinafter, referred to as a “baseportion”) having a height lower than that of the portion described aboveare normally present on the surface of the magnetic layer. The inventorshave thought that the spacing described above is a value which is anindex of a distance between the head and the base portion in a casewhere the magnetic tape and the head slide on each other. However, it isthought that, in a case where fatty acid ester included in the magneticlayer forms a liquid film on the surface of the magnetic layer, theliquid film is present between the base portion and the head, and thus,the spacing is narrowed by the thickness of the liquid film.

Meanwhile, the lubricant is generally divided broadly into a fluidlubricant and a boundary lubricant and fatty acid ester is known as acomponent which can function as a liquid lubricant. It is consideredthat fatty acid ester can protect the magnetic layer by forming a liquidfilm on the surface of the magnetic layer. The inventors have thoughtthat the protection of the surface of the magnetic layer due to thepresence of the liquid film of fatty acid ester contributes topreventing the surface of the magnetic layer from chipping due to thesliding on the head, and preventing cut scraps from becoming headattached materials. However, it is thought that an excessive amount offatty acid ester present on the surface of the magnetic layer causessticking due to strong adhesiveness due to formation of a meniscus(liquid crosslinking) between the surface of the magnetic layer and thehead due to fatty acid ester.

In regards to this point, the inventors focused on the idea that fattyacid ester is a component having properties of volatilizing by vacuumheating, and the difference (S_(after)−S_(before)) of a spacing betweena state after the vacuum heating (state in which a liquid film of fattyacid ester is volatilized and removed) and a state before the vacuumheating (state in which the liquid film of fatty acid ester is present)was used as an index for the thickness of the liquid film formed offatty acid ester on the surface of the magnetic layer. The inventorshave surmised that the presence of the liquid film of fatty acid esteron the surface of the magnetic layer, so that the value of thedifference is greater than 0 nm and equal to or smaller than 8.0 nm,prevents generation of cut scraps due to chipping of the surface of themagnetic layer due to sliding on the head, while preventing sticking.From a viewpoint of further preventing chipping of the surface of themagnetic layer, the difference (S_(after)−S_(before)) is preferablyequal to or greater than 0.1 nm, more preferably equal to or greaterthan 1.0 nm, even more preferably equal to or greater than 1.5 nm, stillpreferably equal to or greater than 2.0 nm, and still more preferablyequal to or greater than 2.5 nm. Meanwhile, from a viewpoint of furtherpreventing occurrence of sticking, the difference (S_(after)−S_(before))is preferably equal to or smaller than 7.5 nm, more preferably equal toor smaller than 7.0 nm, even more preferably equal to or smaller than6.5 nm, still preferably equal to or smaller than 6.0 nm, still morepreferably equal to or smaller than 5.0 nm, and still even morepreferably equal to or smaller than 4.5 nm. The difference(S_(after)−S_(before)) can be controlled by the amount of fatty acidester added to a magnetic layer forming composition. In addition,regarding the magnetic tape including a non-magnetic layer between thenon-magnetic support and the magnetic layer, the difference(S_(after)−S_(before)) can be controlled by the amount of fatty acidester added to a non-magnetic layer forming composition. This is becausethat the non-magnetic layer can play a role of holding a lubricant andsupplying the lubricant to the magnetic layer, and fatty acid esterincluded in the non-magnetic layer may be moved to the magnetic layerand present in the surface of the magnetic layer.

FWHM_(before) and FWHM_(after)

A smaller value of the full width at half maximum of spacingdistribution means that a variation in the values of the spacingmeasured on each part of the surface of the magnetic layer is small. Asa result of intensive studies, the inventors have thought that, in orderto prevent sticking between the surface of the magnetic layer of themagnetic tape and the head and occurrence of chipping of the surface ofthe magnetic layer during the running, it is effective to increaseuniformity of a contact state between the surface of the magnetic layerand the head, by increasing uniformity of a height of projectionspresent on the surface of the magnetic layer and increasing uniformityof a thickness of a liquid film of fatty acid ester. The inventors havethought that an increase in uniformity of the contact state between thesurface of the magnetic layer and the head is also effective toimprovement of electromagnetic conversion characteristics, because adecrease in electromagnetic conversion characteristics due to spacingvariation is prevented.

In regards to this point, it is considered that the reason for thevariation in values of the spacing is a variation in height of theprojection of the surface of the magnetic layer and a variation in thethickness of the liquid film fatty acid ester. The inventors havesurmised that the full width at half maximum of the spacing distributionFWHM_(before) measured before the vacuum heating, that is, in a statewhere the liquid film of fatty acid ester is present on the surface ofthe magnetic layer, becomes great, as the variation in height of theprojection and the variation in the thickness of the liquid film offatty acid ester are great. Particularly, the spacing distributionFWHM_(before) is greatly affected by the variation in the thickness ofthe liquid film of fatty acid ester. In contrast, the inventors havesurmised that the full width at half maximum of the spacing distributionFWHM_(after) measured after the vacuum heating, that is, in a statewhere the liquid film of fatty acid ester is removed from the surface ofthe magnetic layer, becomes great, as the variation in height of theprojection is great. That is, the inventors have surmised that smallfull widths at half maximum of spacing distributions FWHM_(before) andFWHM_(after) mean a small variation in the thickness of the liquid filmof fatty acid ester on the surface of the magnetic layer and a smallvariation in height of the projection. The inventors have thought thatit is possible to prevent sticking between the surface of the magneticlayer of the magnetic tape and the head and occurrence of chipping ofthe surface of the magnetic layer during running, and improveelectromagnetic conversion characteristics, by increasing the uniformityof the height of the projection and the thickness of the liquid film offatty acid ester so that the full widths at half maximum of the spacingdistributions FWHM_(before) and FWHM_(after) are greater than 0 nm andequal to or smaller than 7.0 nm.

Both of the full width at half maximum of spacing distributionFWHM_(before) before the vacuum heating and the full width at halfmaximum of spacing distribution FWHM_(after) after the vacuum heatingwhich are measured in the magnetic tape are greater than 0 nm and equalto or smaller than 7.0 nm. It is thought that this point contributes tothe prevention of sticking of the surface of the magnetic layer of themagnetic tape and the head and occurrence of chipping of the surface ofthe magnetic layer during running. From a viewpoint of furtherpreventing the sticking and occurrence of chipping of the surface of themagnetic layer, the FWHM_(before) and the FWHM_(after) are preferablyequal to or smaller than 6.5 nm, more preferably equal to or smallerthan 6.0 nm, even more preferably equal to or smaller than 5.5 nm, stillmore preferably equal to or smaller than 5.0 nm, and still even morepreferably equal to or smaller than 4.5 nm. The FWHM_(before) and theFWHM_(after) can be, for example, equal to or greater than 0.5 nm, equalto or greater than 1.0 nm, equal to or greater than 2.0 nm, or equal toor greater than 3.0 nm. Meanwhile, from a viewpoint of preventing thesticking and occurrence of chipping of the surface of the magneticlayer, it is preferable that the values thereof are small, andtherefore, the values thereof may be smaller than the exemplifiedvalues.

The FWHM_(before) measured before the vacuum heating can be decreasedmainly by decreasing the variation in the thickness of the liquid filmof fatty acid ester. An example of a specific method will be describedlater. Meanwhile, the FWHM_(after) measured after the vacuum heating canbe decreased by decreasing the variation in height of the projection ofthe surface of the magnetic layer. In order to realize the decreasedescribed above, it is preferable that a presence state of the powdercomponent included in the magnetic layer, for example, non-magneticfiller, which will be described later specifically, in the magneticlayer is controlled. An example of a specific method will be describedlater.

Next, the magnetic layer and the like of the magnetic tape will bedescribed more specifically.

Magnetic Layer

Fatty Acid Ester

The magnetic tape includes fatty acid ester in the magnetic layer. Thefatty acid ester may be included alone as one type or two or more typesthereof may be included. Examples of fatty acid ester include esters oflauric acid, myristic acid, palmitic acid, stearic acid, oleic acid,linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidicacid. Specific examples thereof include butyl myristate, butylpalmitate, butyl stearate (butyl stearate), neopentyl glycol dioleate,sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyloleate, isocetyl stearate, isotridecyl stearate, octyl stearate,isooctyl stearate, amyl stearate, and butoxyethyl stearate.

The content of fatty acid ester, as the content of the magnetic layerforming composition, is, for example, 0.1 to 10.0 parts by mass and ispreferably 1.0 to 7.0 parts by mass with respect to 100.0 parts by massof ferromagnetic powder. In a case of using two or more kinds ofdifferent fatty acid esters as the fatty acid ester, the content thereofis the total content thereof. In the invention and the specification,the same applies to content of other components, unless otherwise noted.In addition, in the invention and the specification, a given componentmay be used alone or used in combination of two or more kinds thereof,unless otherwise noted.

In a case where the magnetic tape includes a non-magnetic layer betweenthe non-magnetic support and the magnetic layer, the content of fattyacid ester in a non-magnetic layer forming composition is, for example,0 to 10.0 parts by mass and is preferably 0.1 to 8.0 parts by mass withrespect to 100.0 parts by mass of non-magnetic powder.

Other Lubricants

The magnetic tape includes fatty acid ester which is one kind oflubricants at least in the magnetic layer. The lubricants other thanfatty acid ester may be arbitrarily included in the magnetic layerand/or the non-magnetic layer. As described above, the lubricantincluded in the non-magnetic layer may be moved to the magnetic layerand present in the surface of the magnetic layer. As the lubricant whichmay be arbitrarily included, fatty acid can be used. In addition, fattyacid amide and the like can also be used. Fatty acid ester is known as acomponent which can function as a fluid lubricant, whereas fatty acidand fatty acid amide are known as a component which can function as aboundary lubricant. It is considered that the boundary lubricant is alubricant which can be adsorbed to a surface of powder (for example,ferromagnetic powder) and form a rigid lubricant film to decreasecontact friction.

Examples of fatty acid include lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenicacid, erucic acid, and elaidic acid, and stearic acid, myristic acid,and palmitic acid are preferable, and stearic acid is more preferable.Fatty acid may be included in the magnetic layer in a state of salt suchas metal salt.

As fatty acid amide, amide of various fatty acid described above isused, and examples thereof include lauric acid amide, myristic acidamide, palmitic acid amide, and stearic acid amide.

Regarding fatty acid and a derivative of fatty acid (amide and ester), apart derived from fatty acid of the fatty acid derivative preferably hasa structure which is the same as or similar to that of fatty acid usedin combination. As an example, in a case of using stearic acid as fattyacid, it is preferable to use stearic acid ester and/or stearic acidamide.

The content of fatty acid in the magnetic layer forming composition is,for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts bymass, and more preferably 1.0 to 7.0 parts by mass, with respect to100.0 parts by mass of the ferromagnetic powder. The content of fattyacid amide in the magnetic layer forming composition is, for example, 0to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and morepreferably 0 to 1.0 part by mass with respect to 100.0 parts by mass ofthe ferromagnetic powder.

In a case where the magnetic tape includes a non-magnetic layer betweenthe non-magnetic support and the magnetic layer, the content of fattyacid in the non-magnetic layer forming composition is, for example, 0 to10.0 parts by mass, preferably 1.0 to 10.0 parts by mass, and morepreferably 1.0 to 7.0 parts by mass with respect to 100.0 parts by massof the non-magnetic powder. The content of fatty acid amide in thenon-magnetic layer forming composition is, for example, 0 to 3.0 partsby mass and preferably 0 to 1.0 part by mass with respect to 100.0 partsby mass of the non-magnetic powder.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer,ferromagnetic powder normally used in the magnetic layer of variousmagnetic recording media can be used. It is preferable to useferromagnetic powder having a small average particle size, from aviewpoint of improvement of recording density of the magnetic tape. Fromthis viewpoint, ferromagnetic powder having an average particle sizeequal to or smaller than 50 nm is preferably used as the ferromagneticpowder. Meanwhile, the average particle size of the ferromagnetic powderis preferably equal to or greater than 10 nm, from a viewpoint ofstability of magnetization.

As a preferred specific example of the ferromagnetic powder,ferromagnetic hexagonal ferrite powder can be used. An average particlesize of the ferromagnetic hexagonal ferrite powder is preferably 10 nmto 50 nm and more preferably 20 nm to 50 nm, from a viewpoint ofimprovement of recording density and stability of magnetization. Fordetails of the ferromagnetic hexagonal ferrite powder, descriptionsdisclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726Acan be referred to, for example.

As a preferred specific example of the ferromagnetic powder,ferromagnetic metal powder can also be used. An average particle size ofthe ferromagnetic metal powder is preferably 10 nm to 50 nm and morepreferably 20 nm to 50 nm, from a viewpoint of improvement of recordingdensity and stability of magnetization. For details of the ferromagneticmetal powder, descriptions disclosed in paragraphs 0137 to 0141 ofJP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can bereferred to, for example.

In the invention and the specification, average particle sizes ofvarious powder such as the ferromagnetic powder and the like are valuesmeasured by the following method with a transmission electronmicroscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with atransmission electron microscope, the image is printed on printing paperso that the total magnification of 500,000 to obtain an image ofparticles configuring the powder. A target particle is selected from theobtained image of particles, an outline of the particle is traced with adigitizer, and a size of the particle (primary particle) is measured.The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particlesarbitrarily extracted. An arithmetical mean of the particle size of 500particles obtained as described above is an average particle size of thepowder. As the transmission electron microscope, a transmission electronmicroscope H-9000 manufactured by Hitachi, Ltd. can be used, forexample. In addition, the measurement of the particle size can beperformed by well-known image analysis software, for example, imageanalysis software KS-400 manufactured by Carl Zeiss. The averageparticle size shown in examples which will be described later is a valuemeasured by using transmission electron microscope H-9000 manufacturedby Hitachi, Ltd. as the transmission electron microscope, and imageanalysis software KS-400 manufactured by Carl Zeiss as the imageanalysis software, unless otherwise noted. In the invention and thespecification, the powder means an aggregate of a plurality ofparticles. For example, the ferromagnetic powder means an aggregate of aplurality of ferromagnetic particles. The aggregate of the plurality ofparticles not only includes an aspect in which particles configuring theaggregate directly come into contact with each other, and also includesan aspect in which a binding agent or an additive which will bedescribed later is interposed between the particles. A term “particles”is also used for describing the powder.

As a method of collecting a sample powder from the magnetic tape inorder to measure the particle size, a method disclosed in a paragraph of0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in acase where the shape of the particle observed in the particle imagedescribed above is a needle shape, a fusiform shape, or a columnar shape(here, a height is greater than a maximum long diameter of a bottomsurface), the size (particle size) of the particles configuring thepowder is shown as a length of a long axis configuring the particle,that is, a long axis length, (2) in a case where the shape of theparticle is a planar shape or a columnar shape (here, a thickness or aheight is smaller than a maximum long diameter of a plate surface or abottom surface), the particle size is shown as a maximum long diameterof the plate surface or the bottom surface, and (3) in a case where theshape of the particle is a sphere shape, a polyhedron shape, or anunspecified shape, and the long axis configuring the particles cannot bespecified from the shape, the particle size is shown as an equivalentcircle diameter. The equivalent circle diameter is a value obtained by acircle projection method.

In addition, regarding an average acicular ratio of the powder, a lengthof a short axis, that is, a short axis length of the particles ismeasured in the measurement described above, a value of (long axislength/short axis length) of each particle is obtained, and anarithmetical mean of the values obtained regarding 500 particles iscalculated. Here, unless otherwise noted, in a case of (1), the shortaxis length as the definition of the particle size is a length of ashort axis configuring the particle, in a case of (2), the short axislength is a thickness or a height, and in a case of (3), the long axisand the short axis are not distinguished, thus, the value of (long axislength/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of theparticle is specified, for example, in a case of definition of theparticle size (1), the average particle size is an average long axislength, in a case of the definition (2), the average particle size is anaverage plate diameter, and an average plate ratio is an arithmeticalmean of (maximum long diameter/thickness or height). In a case of thedefinition (3), the average particle size is an average diameter (alsoreferred to as an average particle diameter).

The content (filling percentage) of the ferromagnetic powder of themagnetic layer is preferably 50 to 90 mass % and more preferably 60 to90 mass %. The components other than the ferromagnetic powder of themagnetic layer are at least a binding agent and fatty acid ester and oneor more kinds of additives may be arbitrarily included. A high fillingpercentage of the ferromagnetic powder in the magnetic layer ispreferable from a viewpoint of improvement recording density.

Binding Agent

The magnetic tape is a coating type magnetic tape, and the magneticlayer includes a binding agent. As the binding agent, one or more kindsof resin is used. The resin may be a homopolymer or a copolymer. As thebinding agent, various resins normally used as a binding agent of thecoating type magnetic recording medium can be used. For example, as thebinding agent, a resin selected from a polyurethane resin, a polyesterresin, a polyamide resin, a vinyl chloride resin, an acrylic resinobtained by copolymerizing styrene, acrylonitrile, or methylmethacrylate, a cellulose resin such as nitrocellulose, an epoxy resin,a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetalor polyvinyl butyral can be used alone or a plurality of resins can bemixed with each other to be used. Among these, a polyurethane resin, anacrylic resin, a cellulose resin, and a vinyl chloride resin arepreferable. These resins can be used as the binding agent even in thenon-magnetic layer and/or a back coating layer which will be describedlater. For the binding agent described above, description disclosed inparagraphs 0028 to 0031 of JP2010-24113A can be referred to. Inaddition, the binding agent may be a radiation curable resin such as anelectron beam-curable resin. For the radiation curable resin,descriptions disclosed in paragraphs 0044 and 0045 of JP2011-048878A canbe referred to.

An average molecular weight of the resin used as the binding agent canbe, for example, 10,000 to 200,000 as a weight-average molecular weight.The weight-average molecular weight in the invention and thespecification is a value obtained by performing polystyrene conversionof a value measured by gel permeation chromatography (GPC). As themeasurement conditions, the following conditions can be used.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In addition, a curing agent can also be used together with the bindingagent. As the curing agent, in one aspect, a thermosetting compoundwhich is a compound in which a curing reaction (crosslinking reaction)proceeds due to heating can be used, and in another aspect, aphotocurable compound in which a curing reaction (crosslinking reaction)proceeds due to light irradiation can be used. At least a part of thecuring agent is included in the magnetic layer in a state of beingreacted (crosslinked) with other components such as the binding agent,by proceeding the curing reaction in the magnetic layer forming step.The preferred curing agent is a thermosetting compound, polyisocyanateis suitable. For details of the polyisocyanate, descriptions disclosedin paragraphs 0124 and 0125 of JP2011-216149A can be referred to, forexample. The amount of the curing agent can be, for example, 0 to 80.0parts by mass with respect to 100.0 parts by mass of the binding agentin the magnetic layer forming composition, and is preferably 50.0 to80.0 parts by mass, from a viewpoint of improvement of strength of eachlayer such as the magnetic layer.

Other Components

Additives can be added to the magnetic layer, if necessary. It ispreferable that the magnetic layer includes one or more kinds of thenon-magnetic filler. The non-magnetic filler is identical to thenon-magnetic powder. As the non-magnetic filler, a non-magnetic filler(hereinafter, also referred to as a “projection formation agent”) whichis added for controlling the projection of the surface of the magneticlayer and a non-magnetic filler (hereinafter, referred to as an“abrasive”) which is added as an abrasive imparting abrasive propertiesto the surface of the magnetic layer are mainly used. The magnetic layerpreferably includes at least the projection formation agent and morepreferably includes the projection formation agent and the abrasive.

The projection formation agent may be powder of inorganic substances(inorganic powder) or powder of organic substances (organic powder), andis preferably powder of inorganic substances. In addition, carbon blackis also preferable. An average particle size of carbon black ispreferably equal to or greater than 20 nm and more preferably equal toor greater than 30 nm. In addition, the average particle size of carbonblack is preferably equal to or smaller than 150 nm and more preferablyequal to or smaller than 100 nm.

Examples of the inorganic powder include powder of inorganic oxide suchas metal oxide, metal carbonate, metal sulfate, metal nitride, metalcarbide, and metal sulfide, and specific examples thereof include powderof α-alumina, β-alumina, γ-alumina, θ-alumina, silicon oxide such assilicon dioxide, silicon carbide, chromium oxide, cerium oxide, α-ironoxide, goethite, corundum, silicon nitride, titanium carbide, titaniumdioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide,boron nitride, zinc oxide, calcium carbonate, calcium sulfate, bariumsulfate, and molybdenum disulfide, or composite inorganic powderincluding two or more kinds thereof. The inorganic oxide powder is morepreferable and silicon oxide powder is even more preferable.

The projection formation agent preferably has uniformity of the particlesize, from a viewpoint of further improving electromagnetic conversioncharacteristics. From a viewpoint of availability of particles havinghigh uniformity of the particle size, the projection formation agent ispreferably colloidal particles. In the invention and the specification,the “colloidal particles” are particles which are not precipitated anddispersed to generate a colloidal dispersion, in a case where 1 g of theparticles is added to 100 mL of at least one organic solvent of at leastmethyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or amixed solvent including two or more kinds of the solvent described aboveat an arbitrary mixing ratio. The average particle size of the colloidalparticles is a value obtained by a method disclosed in a paragraph 0015of JP2011-048878A as a measurement method of an average particlediameter. In a case where the non-magnetic filler used in the formationof the magnetic layer is available, it is possible to determine whetheror not the non-magnetic filler included in the magnetic layer iscolloidal particles, by evaluating whether or not such a non-magneticfiller has properties which are the properties of the colloidalparticles described above. Alternatively, the determination can also beperformed by evaluating whether or not the non-magnetic filler extractedfrom the magnetic layer has properties which are the properties of thecolloidal particles described above. The extraction of the non-magneticfiller from the magnetic layer can be performed by the following method,for example.

1. 1 g of the magnetic layer is scraped off. The scraping can beperformed, for example, by a razor blade.

2. A magnetic layer sample obtained by the scraping is put in a vesselsuch as an eggplant flask and 100 ml of tetrahydrofuran is added intothis vessel. Examples of tetrahydrofuran include commercially availabletetrahydrofuran to which a stabilizer is added and commerciallyavailable tetrahydrofuran to which a stabilizer is not added. Meanwhile,here, the commercially available tetrahydrofuran to which a stabilizeris not added is used. The same applies to tetrahydrofuran used inwashing described hereinafter.

3. A reflux tube is attached to the vessel and heated in a water bath ata water temperature of 60° C. for 90 minutes. After filtering thecontent in the heated vessel with a filter paper, the solid contentremaining on the filter paper is washed with tetrahydrofuran severaltimes, and the washed solid content is moved to a vessel such as abeaker. A 4 N (4 mol/L) hydrochloric acid aqueous solution is added intothis vessel and a residue remaining without being dissolved is extractedby filtering. As a filter, a filter having a hole diameter smaller than0.05 μm is used. For example, a membrane filter used for chromatographyanalysis (for example, MF Millipore manufactured by Merck MilliporeCorporation) can be used. The residue extracted by the filtering iswashed with pure water several times and dried.

Ferromagnetic powder and organic matters (binding agent and the like)are dissolved by the operation described above, and a non-magneticfiller is collected as a residue.

By performing the steps described above, the non-magnetic filler can beextracted from the magnetic layer. In a case where a plurality of kindsof non-magnetic fillers are included in the non-magnetic fillerextracted as described above, the plurality of kinds of non-magneticfillers can be divided depending on the differences of density.

As preferred colloidal particles, inorganic oxide colloidal particlescan be used. As the inorganic oxide colloidal particles, colloidalparticles of inorganic oxide described above can be used, and compositeinorganic oxide colloidal particles such as SiO₂.Al₂O₃, SiO₂.B₂O₃,TiO₂.CeO₂, SnO₂.Sb₂O₃, SiO₂.Al₂O₃.TiO₂, and TiO₂.CeO₂.SiO₂ can be used.The inorganic oxide colloidal particles such as SiO₂, Al₂O₃, TiO₂, ZrO₂,and Fe₂O₃ are preferable and silica colloidal particles (colloidalsilica) are particularly preferable. Meanwhile, typical colloidalparticles have a hydrophilic surface, and thus, the colloidal particlesare suitable for manufacturing a colloidal solution using water as adispersion medium. For example, colloidal silica obtained by a generalsynthesis method has a surface covered with polarized oxygen atoms(O²⁻), and thus, colloidal silica adsorbs water in water, forms ahydroxyl group, and is stabilized. However, these particles are hardlypresent in a colloidal state, in an organic solvent normally used in themagnetic layer forming composition. With respect to this, the colloidalparticles of the invention and the specification are particles which arenot precipitated but are dispersed to cause a colloidal dispersion, when1 g thereof is added with respect to 100 mL of the organic solventdescribed above. Such colloidal particles can be prepared by awell-known method of hydrophobicizing the surface by surface treatment.For details of such hydrophobization treatment, descriptions disclosedin JP1993-269365A (JP-H05-269365A), JP1993-287213A (JP-H05-287213A), andJP2007-63117A are referred to.

As a manufacturing method of the silica colloidal particles (colloidalsilica) which are preferred colloidal particles, two kinds of methodssuch as a water glass method and a sol-gel method are generally known.The water glass method is a method of using silica soda (sodiumsilicate, so-called water glass) as a raw material, performing ionexchange with respect to this to generate an active silica, and causingparticle growth. Meanwhile, the sol-gel method is a method of usingtetraalkoxysilane as a raw material, and performing hydrolysis under abasic catalyst and causing particle growth at the same time. In a caseof using the silica colloidal particles, the silica colloidal particlesmay be manufactured by any manufacturing method described above.

An average particle size of the projection formation agent is preferably50 to 200 nm, and more preferably 50 to 150 nm.

The content of the projection formation agent in the magnetic layer ispreferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 partsby mass with respect to 100.0 parts by mass of the ferromagnetic powder.

Meanwhile, the abrasive may be inorganic powder or organic powder, andthe inorganic powder is preferable. Examples of the abrasive includepowders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂,TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), ironoxide, diamond, and the like which are materials normally used as theabrasive of the magnetic layer, and among these, alumina powder such asα-alumina and silicon carbide powder are preferable. The content of theabrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass,more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0to 10.0 parts by mass with respect to 100.0 parts by mass of theferromagnetic powder. The average particle size of the abrasive is, forexample, 30 to 300 nm and preferably 50 to 200 nm.

An arbitrary amount of one or more kinds of various additives such as adispersing agent, a dispersing assistant, an antifungal agent, anantistatic agent, an antioxidant, and carbon black may be further addedto the magnetic layer. As the additives, commercially available productscan be suitably selectively used according to the desired properties.Alternatively, a compound synthesized by a well-known method can be usedas the additives.

The magnetic layer described above can be directly provided on thenon-magnetic support or on a non-magnetic layer which is formed on thenon-magnetic support.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape mayinclude a magnetic layer directly on a non-magnetic support, or mayinclude a non-magnetic layer including non-magnetic powder and a bindingagent between the non-magnetic support and the magnetic layer. Thenon-magnetic powder used in the non-magnetic layer may be inorganicpowder or organic powder. In addition, carbon black and the like can beused. Examples of the inorganic powder include powder of metal, metaloxide, metal carbonate, metal sulfate, metal nitride, metal carbide, andmetal sulfide. These non-magnetic powder can be purchased as acommercially available product or can be manufactured by a well-knownmethod. For details thereof, descriptions disclosed in paragraphs 0146to 0150 of JP2011-216149A can be referred to. For carbon black which canbe used in the non-magnetic layer, descriptions disclosed in paragraphs0040 and 0041 of JP2010-24113A can be referred to. The content (fillingpercentage) of the non-magnetic powder of the non-magnetic layer ispreferably 50 to 90 mass % and more preferably 60 to 90 mass %.

In regards to other details of a binding agent or additives of thenon-magnetic layer, the well-known technology regarding the non-magneticlayer can be applied. In addition, in regards to the type and thecontent of the binding agent, and the type and the content of theadditive, for example, the well-known technology regarding the magneticlayer can be applied.

The non-magnetic layer of the invention and the specification alsoincludes a substantially non-magnetic layer including a small amount offerromagnetic powder as impurities or intentionally, together with thenon-magnetic powder. Here, the substantially non-magnetic layer is alayer having a residual magnetic flux density equal to or smaller than10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m (100Oe), or a layer having a residual magnetic flux density equal to orsmaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have aresidual magnetic flux density and coercivity.

Back Coating Layer

The magnetic tape can also include a back coating layer includingnon-magnetic powder and a binding agent on a surface side of thenon-magnetic support opposite to the surface provided with the magneticlayer. The back coating layer preferably includes any one or both ofcarbon black and inorganic powder. In regards to the binding agentincluded in the back coating layer and various additives which can bearbitrarily included in the back coating layer, a well-known technologyregarding the treatment of the magnetic layer and/or the non-magneticlayer can be applied.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to asa “support”) will be described. As the non-magnetic support, well-knowncomponents such as polyethylene terephthalate, polyethylene naphthalate,polyamide, polyamide imide, aromatic polyamide subjected to biaxialstretching are used. Among these, polyethylene terephthalate,polyethylene naphthalate, and polyamide are preferable. Coronadischarge, plasma treatment, easy-bonding treatment, or heatingtreatment may be performed with respect to these supports in advance.

Various Thickness

A thickness of the non-magnetic support is preferably 3.00 to 6.00 μm.

A thickness of the magnetic layer is preferably equal to or smaller than0.15 μm and more preferably equal to or smaller than 0.10 μm, from aviewpoint of realization of high-density recording required in recentyears. The thickness of the magnetic layer is even more preferably 0.01to 0.10 μm. The magnetic layer may be at least single layer, themagnetic layer may be separated into two or more layers having differentmagnetic properties, and a configuration of a well-known multilayeredmagnetic layer can be applied. A thickness of the magnetic layer in acase where the magnetic layer is separated into two or more layers is atotal thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μmand is preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smallerthan 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic tape and thenon-magnetic support can be acquired by a well-known film thicknessmeasurement method. As an example, a cross section of the magnetic tapein a thickness direction is, for example, exposed by a well-known methodof ion beams or microtome, and the exposed cross section is observedwith a scanning electron microscope. In the cross section observation,various thicknesses can be acquired as a thickness acquired at oneposition of the cross section in the thickness direction, or anarithmetical mean of thicknesses acquired at a plurality of positions oftwo or more positions, for example, two positions which are arbitrarilyextracted. In addition, the thickness of each layer may be acquired as adesigned thickness calculated according to the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Each composition for forming the magnetic layer, the non-magnetic layer,or the back coating layer normally includes a solvent, together withvarious components described above. As the solvent, various organicsolvents generally used for manufacturing a coating type magneticrecording medium can be used. Among those, from a viewpoint ofsolubility of the binding agent normally used in the coating typemagnetic recording medium, each layer forming composition preferablyincludes one or more ketone solvents such as acetone, methyl ethylketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone,isophorone, and tetrahydrofuran. The amount of the solvent of each layerforming composition is not particularly limited, and can be set to bethe same as that of each layer forming composition of a typical coatingtype magnetic recording medium. In addition, steps of preparing eachlayer forming composition generally include at least a kneading step, adispersing step, and a mixing step provided before and after thesesteps, if necessary. Each step may be divided into two or more stages.All of raw materials used in the invention may be added at an initialstage or in a middle stage of each step. In addition, each raw materialmay be separately added in two or more steps. In the preparation of themagnetic layer forming composition, it is preferable that theferromagnetic powder and the abrasive are separately dispersed asdescribed above. In addition, in order to manufacture the magnetic tape,a well-known manufacturing technology can be used. In the kneading step,an open kneader, a continuous kneader, a pressure kneader, or a kneaderhaving a strong kneading force such as an extruder is preferably used.The details of the kneading processes of these kneaders are disclosed inJP1989-106338A (JP-H01-106338A) and JP 1989-79274A (JP-H01-79274A). Inaddition, in order to disperse each layer forming composition, glassbeads and one or more kinds of other dispersion beads can be used as adispersion medium. As such dispersion beads, zirconia beads, titaniabeads, and steel beads which are dispersion beads having high specificgravity are suitable. The dispersion beads can be used by optimizing abead diameter and a filling percentage of the dispersion beads. As adispersing machine, a well-known dispersing machine can be used. Eachlayer forming composition may be filtered by a well-known method beforeperforming the coating step. The filtering can be performed by using afilter, for example. As the filter used in the filtering, a filterhaving a hole diameter of 0.01 to 3 μm can be used, for example.

In addition, the FWHM_(after) measured after the vacuum heating tends tobe decreased, in a case where the dispersion conditions of the magneticlayer forming composition are reinforced (for example, the number oftimes of the dispersion is increased, the dispersion time is extended,and the like), and/or the filtering conditions are reinforced (forexample, a filter having a small hole diameter is used as a filter usedin the filtering, the number of times of the filtering is increased, andthe like). The inventors have surmised that this is because theuniformity of the height of the projection present on the surface of themagnetic layer is improved, by improving dispersibility and/or theuniformity of the particle size of the powder included in the magneticlayer forming composition, particularly, the non-magnetic filler whichmay function as the projection formation agent described above. In oneaspect, a preliminary experiment can be performed before the actualmanufacturing, and the dispersion conditions and/or the filteringconditions can be optimized.

In addition, in the magnetic tape including the magnetic layer includingcarbon black as the non-magnetic filler, it is effective to use thedispersing agent for improving dispersibility of the carbon black as amagnetic layer component, in order to decrease the FWHM_(after) measuredafter the vacuum heating. For example, organic tertiary amine can beused as a dispersing agent of carbon black. For details of the organictertiary amine, descriptions disclosed in paragraphs 0011 to 0018 and0021 of JP2013-049832A can be referred to. The organic tertiary amine ismore preferably trialkylamine. An alkyl group included in trialkylamineis preferably an alkyl group having 1 to 18 carbon atoms. Three alkylgroups included in trialkylamine may be the same as each other ordifferent from each other. For details of the alkyl group, descriptionsdisclosed in paragraphs 0015 and 0016 of JP2013-049832A can be referredto. As trialkylamine, trioctylamine is particularly preferable.

Coating Step, Cooling Step, Heating and Drying Step, BurnishingTreatment Step, Curing Step, and Vibration Imparting Step

The magnetic layer can be formed by directly applying the magnetic layerforming composition onto the non-magnetic support or performingmultilayer coating of the magnetic layer forming composition with thenon-magnetic layer forming composition in order or at the same time. Fordetails of the coating for forming each layer, a description disclosedin a paragraph 0066 of JP2010-231843A can be referred to.

In a preferred aspect, a magnetic layer can be formed through a magneticlayer forming step including a coating step of applying a magnetic layerforming composition including ferromagnetic powder, a binding agent,fatty acid ester, a curing agent, and a solvent onto a non-magneticsupport directly or with another layer interposed therebetween, to forma coating layer, a heating and drying step of drying the coating layerby a heating process, and a curing step of performing a curing processwith respect to the coating layer. The magnetic layer forming steppreferably includes a cooling step of cooling the coating layer betweenthe coating step and the heating and drying step, and more preferablyincludes a burnishing treatment step of performing a burnishingtreatment with respect to the surface of the coating layer between theheating and drying step and the curing step.

The inventors have thought that it is preferable that the cooling stepand the burnishing treatment step in the magnetic layer forming step, inorder to set the logarithmic decrement to be equal to or smaller than0.050. More specific description is as follows.

The inventors have surmised that performing the cooling step of coolingthe coating layer between the coating step and the heating and dryingstep contributes to causing pressure sensitive adhesive componentseparated from the surface of the magnetic layer in a case where thehead comes into contact with and slides on the surface of the magneticlayer, to be localized in the surface and/or a surface layer part in thevicinity of the surface of the coating layer. The inventors havesurmised that this is because the pressure sensitive adhesive componentat the time of solvent volatilization in the heating and drying step iseasily moved to the surface and/or the surface layer part of the coatinglayer, by cooling the coating layer of the magnetic layer formingcomposition before the heating and drying step. However, the reasonthereof is not clear. In addition, the inventors have thought that thepressure sensitive adhesive component can be removed by performing theburnishing treatment with respect to the surface of the coating layer inwhich the pressure sensitive adhesive component is localized on thesurface and/or surface layer part. The inventors have surmised thatperforming the curing step after removing the pressure sensitiveadhesive component contributes setting the logarithmic decrement to beequal to or smaller than 0.050. However, this is merely a surmise, andthe invention is not limited thereto.

In addition, it is thought that, by performing the vibration impartingstep, the liquid film of fatty acid ester on the surface of the magneticlayer flows and the uniformity of the thickness of the liquid film isimproved. Accordingly, it is preferable to perform the vibrationimparting step in order to control the FWHM_(before).

As described above, multilayer coating of the magnetic layer formingcomposition can be performed with the non-magnetic layer formingcomposition in order or at the same time. In a preferred aspect, themagnetic tape can be manufactured by successive multilayer coating. Amanufacturing step including the successive multilayer coating can bepreferably performed as follows. The non-magnetic layer is formedthrough a coating step of applying a non-magnetic layer formingcomposition onto a non-magnetic support to form a coating layer, and aheating and drying step of drying the formed coating layer by a heatingprocess. In addition, the magnetic layer is formed through a coatingstep of applying a magnetic layer forming composition onto the formednon-magnetic layer to form a coating layer, and a heating and dryingstep of drying the formed coating layer by a heating process.

Hereinafter, a specific aspect of the manufacturing method of themagnetic tape will be described with reference to FIG. 4. However, theinvention is not limited to the following specific aspect.

FIG. 4 is a step schematic view showing a specific aspect of a step ofmanufacturing the magnetic tape including a non-magnetic layer and amagnetic layer in this order on one surface of a non-magnetic supportand including a back coating layer on the other surface thereof. In theaspect shown in FIG. 4, an operation of sending a non-magnetic support(elongated film) from a sending part and winding the non-magneticsupport around a winding part is continuously performed, and variousprocesses of coating, drying, and orientation are performed in each partor each zone shown in FIG. 4, and thus, it is possible to sequentiallyform a non-magnetic layer and a magnetic layer on one surface of therunning non-magnetic support by multilayer coating and to form a backcoating layer on the other surface thereof. Such a manufacturing methodcan be set to be identical to the manufacturing method normallyperformed for manufacturing a coating type magnetic recording medium,except for including a cooling zone in the magnetic layer forming step,including the burnishing treatment step before the curing process, andthe vibration imparting step after the curing process.

The non-magnetic layer forming composition is applied onto thenon-magnetic support sent from the sending part in a first coating part(coating step of non-magnetic layer forming composition).

After the coating step, in a first heating process zone, the coatinglayer of the non-magnetic layer forming composition formed in thecoating step is heated after to dry the coating layer (heating anddrying step). The heating and drying step can be performed by causingthe non-magnetic support including the coating layer of the non-magneticlayer forming composition to pass through the heated atmosphere. Anatmosphere temperature of the heated atmosphere here can be, forexample, approximately 60° to 140°. Here, the atmosphere temperature maybe a temperature at which the solvent is volatilized and the coatinglayer is dried, and the atmosphere temperature is not limited to therange described above. In addition, the heated air may arbitrarily blowto the surface of the coating layer. The points described above are alsoapplied to a heating and drying step of a second heating process zoneand a heating and drying step of a third heating process zone which willbe described later, in the same manner. In addition, in a case where acoating layer is formed by using the non-magnetic layer formingcomposition including a radiation curable resin such as an electronbeam-curable resin as a binding agent, a curing process of irradiatingthe coating layer with radiation such as electron beam can be performed.The same applies to the step of forming other layers.

Next, in a second coating part, the magnetic layer forming compositionis applied onto the non-magnetic layer formed by performing the heatingand drying step in the first heating process zone (coating step ofmagnetic layer forming composition).

After the coating step, a coating layer of the magnetic layer formingcomposition formed in the coating step is cooled in a cooling zone(cooling step). For example, it is possible to perform the cooling stepby allowing the non-magnetic support on which the coating layer isformed on the non-magnetic layer to pass through a cooling atmosphere.An atmosphere temperature of the cooling atmosphere is preferably −10°C. to 0° C. and more preferably −5° C. to 0° C. The time for performingthe cooling step (for example, time while an arbitrary part of thecoating layer is delivered to and sent from the cooling zone(hereinafter, also referred to as a “staying time”)) is not particularlylimited. In a case where the staying time is long, the value oflogarithmic decrement tends to be increased. Thus, the staying time ispreferably adjusted by performing preliminary experiment if necessary,so that the logarithmic decrement equal to or smaller than 0.050 isrealized. In the cooling step, cooled air may blow to the surface of thecoating layer.

After that, while the coating layer of the magnetic layer formingcomposition is wet, an orientation process of the ferromagnetic powderin the coating layer is performed in an orientation zone. For theorientation process, a description disclosed in a paragraph 0052 ofJP2010-24113A can be referred to.

The coating layer after the orientation process is subjected to theheating and drying step in the second heating process zone.

Next, in the third coating part, a back coating layer formingcomposition is applied to a surface of the non-magnetic support on aside opposite to the surface where the non-magnetic layer and themagnetic layer are formed, to form a coating layer (coating step of backcoating layer forming composition). After that, the coating layer isheated and dried in the third heating process zone.

By doing so, it is possible to obtain the magnetic tape including thecoating layer of the magnetic layer forming composition heated and driedon the non-magnetic layer, on one surface side of the non-magneticsupport, and the back coating layer on the other surface side thereof.The magnetic tape obtained here becomes a magnetic tape product afterperforming various processes which will be described later.

The obtained magnetic tape is wound around the winding part, and cut(slit) to have a size of a magnetic tape product. The slitting isperformed by using a well-known cutter.

In the slit magnetic tape, the burnishing treatment is performed withrespect to the surface of the heated and dried coating layer of themagnetic layer forming composition, before performing the curing process(heating and light irradiation) in accordance with the types of thecuring agent included in the magnetic layer forming composition(burnishing treatment step between heating and drying step and curingstep). The inventors have surmised that removing the pressure sensitiveadhesive component transitioned to the surface and/or the surface layerpart of the coating layer cooled in the cooling zone by the burnishingtreatment contributes setting the logarithmic decrement to be equal toor smaller than 0.050. However, this is merely a surmise, and theinvention is not limited thereto.

The burnishing treatment is treatment of rubbing a surface of atreatment target with a member (for example, a polishing tape, or agrinding tool such as a grinding blade or a grinding wheel), and can beperformed in the same manner as the well-known burnishing treatment formanufacturing a coating type magnetic recording medium. However, in therelated art, the burnishing treatment was not performed in a stagebefore the curing step, after performing the cooling step and theheating and drying step. With respect to this, the logarithmic decrementcan be equal to or smaller than 0.050 by performing the burnishingtreatment in the stage described above.

The burnishing treatment can be preferably performed by performing oneor both of rubbing of the surface of the coating layer of the treatmenttarget by a polishing tape (polishing) and rubbing of the surface of thecoating layer of the treatment target by a grinding tool (grinding). Ina case where the magnetic layer forming composition includes anabrasive, it is preferable to use a polishing tape including at leastone of an abrasive having higher Mohs hardness than that of the abrasivedescribed above. As the polishing tape, a commercially available productmay be used and a polishing tape manufactured by a well-known method maybe used. As the grinding tool, a well-known blade such as a fixed blade,a diamond wheel, or a rotary blade, or a grinding blade can be used. Inaddition, a wiping treatment of wiping the surface of the coating layerrubbed by the polishing tape and/or the grinding tool with a wipingmaterial. For details of preferred polishing tape, grinding tool,burnishing treatment, and wiping treatment, descriptions disclosed inparagraphs 0034 to 0048, FIG. 1 and examples of JP1994-52544A(JP-H06-52544A) can be referred to. As the burnishing treatment isreinforced, the value of the logarithmic decrement tends to bedecreased. The burnishing treatment can be reinforced as an abrasivehaving high hardness is used as the abrasive included in the polishingtape, and can be reinforced, as the amount of the abrasive in thepolishing tape is increased. In addition, the burnishing treatment canbe reinforced as a grinding tool having high hardness is used as thegrinding tool. In regards to the burnishing treatment conditions, theburnishing treatment can be reinforced as a sliding speed between thesurface of the coating layer of the treatment target and a member (forexample, a polishing tape or a grinding tool) is increased. The slidingspeed can be increased by increasing one or both of a speed at which themember is moved, and a speed at which the magnetic tape of the treatmenttarget is moved.

After the burnishing treatment (burnishing treatment step), the curingprocess is performed with respect to the coating layer of the magneticlayer forming composition. In the aspect shown in FIG. 4, the coatinglayer of the magnetic layer forming composition is subjected to thesurface smoothing treatment, after the burnishing treatment and beforethe curing process. The surface smoothing treatment is preferablyperformed by a calender process. For details of the calender process,for example, description disclosed in a paragraph 0026 of JP2010-231843Acan be referred to. As the calender process is reinforced, the surfaceof the magnetic tape can be smoothened. The calender process isreinforced, as the surface temperature (calender temperature) of acalender roll is increased and/or as calender pressure is increased.

After that, the curing process according to the type of the curing agentincluded in the coating layer is performed with respect to the coatinglayer of the magnetic layer forming composition (curing step). Thecuring process can be performed by the process according to the type ofthe curing agent included in the coating layer, such as a heatingprocess or light irradiation. The curing process conditions are notparticularly limited, and the curing process conditions may be suitablyset in accordance with the list of the magnetic layer formingcomposition used in the coating layer formation, the type of the curingagent, and the thickness of the coating layer. For example, in a casewhere the coating layer is formed by using the magnetic layer formingcomposition including polyisocyanate as the curing agent, the curingprocess is preferably the heating process. In a case where the curingagent is included in a layer other than the magnetic layer, a curingreaction of the layer can also be promoted by the curing process here.Alternatively, the curing step may be separately provided. After thecuring step, the burnishing treatment may be further performed.

Then, the vibration is imparted to the magnetic layer in a vibrationimparting part (vibration imparting step). A vibration imparting methodis not particularly limited. For example, the vibration can be impartedto the magnetic layer by bringing the surface of the non-magneticsupport on which the magnetic layer is formed on a side opposite to themagnetic layer, into contact with a vibration imparting unit. Thenon-magnetic support on which the magnetic layer is formed may beallowed to run while coming into contact with a vibration impartingunit. The vibration imparting unit, for example, includes an ultrasonicvibrator therein and thus, can impart vibration to a product coming intocontact with the unit. A degree of the vibration imparted to themagnetic layer can be adjusted with a vibration frequency or a strengthof the ultrasonic vibrator and/or a contact time with the vibrationimparting unit. For example, the contact time can be adjusted by arunning speed while the non-magnetic support on which the magnetic layeris formed comes into contact with the vibration imparting unit. Thesevibration imparting conditions are not particularly limited, and may beset so as to control the full width at half maximum of the spacingdistribution described above, particularly, the FWHM_(before) measuredbefore the vacuum heating. In one aspect, a preliminary experiment isperformed before the manufacturing, in order to set the vibrationimparting conditions, and the conditions can be optimized.

By doing so, it is possible to obtain a magnetic tape according to oneaspect of the invention. However, the manufacturing method describedabove is merely an example, the magnetic layer surface roughness Ra, thelogarithmic decrement, the FWHM_(before), and the FWHM_(after), and thedifference (S_(after)−S_(before)) can be respectively controlled to bein the ranges described above by arbitrary methods capable of adjustingthe values thereof, and such an aspect is also included in theinvention.

The magnetic tape according to one aspect of the invention describedabove is generally accommodated in a magnetic tape cartridge and themagnetic tape cartridge is mounted in a drive. The configuration of themagnetic tape cartridge and the drive is well known. The magnetic taperuns (is transported) in the drive, the magnetic head for recordingand/or reproducing of information comes into contact with and slides onthe surface of the magnetic layer, and the recording of the informationon the magnetic tape and/or reproducing of the recorded information areperformed. A running speed of the magnetic tape is also referred to as atransportation speed and is a relative speed of the magnetic tape andthe head at the time of the magnetic tape running. It is preferable thatthe running speed is increased to cause the magnetic tape run at a highspeed, in order to shorten the time necessary for recording informationand/or time necessary for reproducing the recorded information. Fromthis viewpoint, the running speed of the magnetic tape is, for example,preferably equal to or higher than 6.0 m/sec. Meanwhile, it wasdetermined that, in the magnetic tape having the magnetic layer surfaceroughness Ra equal to or smaller than 1.8 nm, a decrease in reproductionoutput occurs, in a case of repeating the high-speed running in theenvironment of a high temperature and high humidity, without anymeasures. With respect to this, in the magnetic tape according to oneaspect of the invention in which the magnetic layer surface roughness Rais equal to or smaller than 1.8 nm and the logarithmic decrement, theFWHM_(before), and the FWHM_(after), and the difference(S_(after)−S_(before)) are in the ranges described above, a decrease inreproduction output during the repeated high-speed running in theenvironment of a high temperature and high humidity can be prevented.

EXAMPLES

Hereinafter, the invention will be described with reference to examples.However, the invention is not limited to aspects shown in the examples.“Parts” and “%” in the following description mean “parts by mass” and“mass %”, unless otherwise noted.

1. Manufacturing of magnetic tape

Example 1

Magnetic Layer Forming Composition

Magnetic Solution

Ferromagnetic hexagonal barium ferrite powder: 100.0 parts

-   -   (coercivity Hc: 2,100 Oe (168 kA/m), average particle size: 25        nm)

Sulfonic acid-containing polyurethane resin: 15.0 parts

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Liquid

α-alumina (average particle size of 110 nm): 9.0 parts

A vinyl chloride copolymer: (MR110 manufactured by Zeon Corporation):0.7 parts

Cyclohexanone: 20.0 parts

Silica Sol

Colloidal silica prepared by a sol-gel method (average particle size:see Table 1): 3.5 parts

Methyl ethyl ketone: 8.2 parts

Other Components

Butyl stearate: 1.0 part

Stearic acid: 1.0 part

Polyisocyanate (CORONATE manufactured by Nippon Polyurethane IndustryCo., Ltd.): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 180.0 parts

Methyl ethyl ketone: 180.0 parts

Non-Magnetic Layer Forming Composition

Non-magnetic inorganic powder (α-iron oxide): 80.0 parts

-   -   (average particle size: 0.15 μm, average acicular ratio: 7,        Brunauer-Emmett-Teller (BET) specific surface area: 52 m²/g)

Carbon black (average particle size of 20 nm): 20.0 parts

An electron beam-curable vinyl chloride copolymer: 13.0 parts

An electron beam-curable polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Cyclohexanone: 140.0 parts

Methyl ethyl ketone: 170.0 parts

Butyl stearate: see Table 1

Stearic acid: 2.0 parts

Back Coating Layer Forming Composition

Non-magnetic inorganic powder (α-iron oxide): 80.0 parts

-   -   (average particle size: 0.15 μm, average acicular ratio: 7, BET        specific surface area: 52 m²/g)

Carbon black (average particle size of 20 nm): 20.0 parts

Carbon black (average particle size of 100 nm): 3.0 parts

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Cyclohexanone: 140.0 parts

Methyl ethyl ketone: 170.0 parts

Stearic acid: 3.0 parts

Polyisocyanate (CORONATE manufactured by Nippon Polyurethane IndustryCo., Ltd.): 5.0 parts

Methyl ethyl ketone: 400.0 parts

Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the followingmethod.

The magnetic solution was kneaded and diluted by an open kneader, andsubjected to a dispersion process of 12 passes, with a transverse beadsmill dispersing device and zirconia (ZrO₂) beads (hereinafter, referredto as “Zr beads”) having a bead diameter of 0.5 mm, by setting a beadfilling percentage as 80 volume %, a circumferential speed of rotordistal end as 10 m/sec, and a retention time for 1 pass as 2 minutes.

After mixing the components described above, the abrasive liquid was putin a vertical sand mill dispersing device together with Zr beads havinga bead diameter of 1 mm, the bead volume/(abrasive liquid volume+beadvolume) was adjusted to be 60%, the sand mill dispersing process wasperformed for 180 minutes, a solution after the process was extracted,and an ultrasonic dispersion filtering process was performed with aflow-type ultrasonic dispersion filtering device.

The magnetic solution, the silica sol, the abrasive liquid, othercomponents, and the finishing additive solvent were introduced into adissolver stirrer, and were stirred at a circumferential speed of 10m/sec for 30 minutes. After that, the treatment was performed with aflow-type ultrasonic dispersing device at a flow rate of 7.5 kg/min forthe number of times of passes shown in Table 1, and then, a magneticlayer forming composition was prepared by performing filtering with afilter having a hole diameter shown in Table 1 for the number of timesshown in Table 1.

The non-magnetic layer forming composition was prepared by the followingmethod.

Each component excluding a lubricant (butyl stearate and stearic acid)was kneaded with an open kneader and diluted, and then, was dispersed byusing a transverse beads mill dispersing device. After that, thelubricant (butyl stearate and stearic acid) was added thereto, andstirred and mixed with a dissolver stirrer, to prepare a non-magneticlayer forming composition.

The back coating layer forming composition was prepared by the followingmethod.

Each component excluding the lubricant (stearic acid), polyisocyanate,and methyl ethyl ketone (400.0 parts) was kneaded with an open kneaderand diluted, and then, was dispersed by using a transverse beads milldispersing device. After that, the lubricant (stearic acid),polyisocyanate, and methyl ethyl ketone (400.0 parts) were addedthereto, and stirred and mixed with a dissolver stirrer, to prepare aback coating layer forming composition.

Manufacturing of Magnetic Tape

A magnetic tape was manufactured by the specific aspect shown in FIG. 4.The magnetic tape was specifically manufactured as follows.

A support made of polyethylene naphthalate having a thickness of 4.50 μmwas sent from the sending part, and the non-magnetic layer formingcomposition was applied to one surface thereof so that the thicknessafter the drying becomes 0.40 μm in the first coating part and was driedin the first heating process zone (atmosphere temperature of 100° C.) toform a coating layer. The formed coating layer was irradiated with anelectron beam with an energy of 40 kGy at an acceleration voltage of 125kV.

Then, the magnetic layer forming composition was applied onto thenon-magnetic layer so that the thickness after the drying becomes 60 nm(0.06 μm) in the second coating part, and a coating layer was formed.The cooling step was performed by passing the formed coating layerthrough the cooling zone in which the atmosphere temperature is adjustedto 0° C. for the staying time shown in Table 1 while the coating layeris wet, a homeotropic alignment process was performed in the orientationzone by applying a magnetic field having a magnetic field strength of0.3 T in a vertical direction, and then, the coating layer was dried inthe second heating process zone (atmosphere temperature of 100° C.).

After that, in the third coating part, the back coating layer formingcomposition was applied to the surface of the support made ofpolyethylene naphthalate on a side opposite to the surface where thenon-magnetic layer and the magnetic layer are formed, so that thethickness after the drying becomes 0.60 μm, to form a coating layer, andthe formed coating layer was dried in the third heating process zone(atmosphere temperature of 100° C.).

The magnetic tape obtained as described above was slit to have a widthof ½ inches (0.0127 meters), and the burnishing treatment and the wipingtreatment were performed with respect to the surface of the coatinglayer of the magnetic layer forming composition. The burnishingtreatment and the wiping treatment were performed by using acommercially available polishing tape (product name: MA22000manufactured by Fujifilm Corporation, abrasive: diamond/Cr₂O₃/red oxide)as the polishing tape, a commercially available sapphire blade(manufactured by Kyocera Corporation, a width of 5 mm, a length of 35mm, and a tip angle of 60 degrees) as the grinding blade, and acommercially available wiping material (product name: WRP736manufactured by Kuraray Co., Ltd.) as the wiping material, in atreatment device having a configuration disclosed in FIG. 1 ofJP1994-52544A (JP-1106-52544A). For the treatment conditions, thetreatment conditions disclosed in Example 12 of JP1994-52544A(JP-H06-52544A).

After the burnishing treatment and the wiping treatment, a calenderprocess (surface smoothing treatment) was performed with a calender rollconfigured of only a metal roll, at a speed of 80 m/min, linear pressureof 300 kg/cm (294 kN/m), and a calender temperature (surface temperatureof a calender roll) shown in Table 1.

After that, a heating process (curing process) was performed in theenvironment of the atmosphere temperature of 70° C. for 36 hours, andthen, the vibration was imparted to the magnetic layer in the vibrationimparting part. Specifically, the support, provided with the magneticlayer, was installed in a vibration imparting device shown in FIG. 5 sothat the surface thereof on a side opposite to the surface where themagnetic layer is formed comes into contact with the vibration impartingunit, and the support provided with the magnetic layer (referencenumeral 1 in FIG. 5) is transported at a transportation speed of 0.5m/sec to impart the vibration to the back coating layer. In FIG. 5, areference numeral 2 denotes a guide roller (a reference numeral 2denotes one of two guide rollers), a reference numeral 3 denotes thevibration imparting device (vibration imparting unit including theultrasonic vibrator), and an arrow denotes a transportation direction.The time from the start of the contact of the arbitrary portion of thesupport, provided with the magnetic layer formed, with the vibrationimparting unit until the end of the contact is shown in Table 1 as thevibration imparting time. The vibration imparting unit used includes anultrasonic vibrator therein. The vibration was imparted by setting avibration frequency and the intensity of the ultrasonic vibrator asvalues shown in Table 1.

By doing so, the magnetic tape of Example 1 was manufactured. Variousthicknesses of the manufactured magnetic tape were obtained by thefollowing method. It was confirmed that the thicknesses of thenon-magnetic support, the formed non-magnetic layer, the magnetic layer,and the back coating layer were the thicknesses described above.

A cross section of the magnetic tape in a thickness direction wasexposed to ion beams and the exposed cross section was observed with ascanning electron microscope. Various thicknesses were obtained as anarithmetical mean of thicknesses obtained at two portions in thethickness direction in the cross section observation.

Examples 2 to 8 and Comparative Examples 1 to 9

A magnetic tape was manufactured by the same method as that in Example1, except that various conditions were changed as shown in Table 1. Thevibration imparting time was adjusted by changing the transportationspeed of the support provided with the magnetic layer.

In Table 1, in the comparative examples in which “not performed” isdisclosed in a column of the cooling zone staying time and a column ofthe burnishing treatment before the curing process, and “performed” isdisclosed in a column of the burnishing treatment after the curingprocess, a magnetic tape was manufactured by a manufacturing step notincluding the cooling zone in the magnetic layer forming step andperforming the burnishing treatment and the wiping treatment by the samemethod as that in Example 1, not before the curing process, but afterthe curing process and before the vibration imparting step.

A part of each magnetic tape of the examples and the comparativeexamples manufactured by the method described above was used in theevaluation described below, and the other part was used in theevaluation of performance which will be described later.

2. Evaluation of Physical Properties of Magnetic Tape

(1) Center Line Average Surface Roughness Ra Measured Regarding Surfaceof Magnetic Layer

The measurement regarding a measurement area of 40 μm×40 μm in thesurface of the magnetic layer of each magnetic tape of the examples andthe comparative examples was performed with an atomic force microscope(AFM, Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tappingmode, and a center line average surface roughness Ra was acquired.RTESP-300 manufactured by BRUKER is used as a probe, a scan speed (probemovement speed) was set as 40 μm/sec, and a resolution was set as 512pixel×512 pixel.

(2) Measurement of Logarithmic Decrement

The logarithmic decrement of the surface of the magnetic layer of themagnetic tape was acquired by the method described above by using arigid-body pendulum type physical properties testing instrumentRPT-3000W manufactured by A&D Company, Limited (pendulum: brass,substrate: glass substrate, a rate of temperature increase of substrate:5° C./min) as the measurement device. A measurement sample cut out fromthe magnetic tape was placed on a glass substrate having a size ofapproximately 3 cm×approximately 5 cm, by being fixed at 4 portions witha fixing tape (Kapton tape manufactured by Du Pont-Toray Co., Ltd.) asshown in FIG. 1. An adsorption time was set as 1 second, a measurementinterval was set as 7 to 10 seconds, a displacement-time curve was drawnregarding the 86-th measurement interval, and the logarithmic decrementwas acquired by using this curve. The measurement was performed in theenvironment of relative humidity of approximately 50%.

(3) FWHM_(before) and FWHM_(after)

The full widths at half maximum FWHM_(before) and FWHM_(after) ofspacing distributions before and after performing the vacuum heatingwere acquired by the following method by using a tape spacing analyzer(TSA) (manufactured by Micro Physics, Inc.).

In a state where a glass sheet included in the TSA was disposed on thesurface of the magnetic layer of the magnetic tape, a hemisphere waspressed against the surface of the back coating layer of the magnetictape at a pressure of 5.05×10⁴ N/m (0.5 atm) by using a hemisphere madeof urethane included in the TSA as a pressing member. In this state, agiven region (150,000 to 200,000 μm²) of the surface of the magneticlayer of the magnetic tape was irradiated with white light from astroboscope included in the TSA through the glass sheet, and theobtained reflected light was received by a charge-coupled device (CCD)through an interference filter (filter selectively passing light at awavelength of 633 nm), and thus, an interference fringe image generatedon the uneven part of the region was obtained.

This image was divided into 300,000 points, a distance (spacing) betweenthe surface of the glass sheet on the magnetic tape side and the surfaceof the magnetic layer of the magnetic tape was acquired, and the fullwidth at half maximum, in a case where this spacing was shown with ahistogram, and this histogram was fit with Gaussian distribution, wasfull width at half maximum of spacing distribution.

The vacuum heating was performed by storing the magnetic tape in avacuum constant temperature drying machine with a degree of vacuum of200 Pa to 0.01 MPa and at inner atmosphere temperature of 70° C. to 90°C. for 24 hours.

(4) Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) was a value obtained bysubtracting a mode of the histogram before the vacuum heating from amode of the histogram after the vacuum heating obtained in the section(3).

3. Evaluation of Performance of Magnetic Tape

(1) Electromagnetic Conversion Characteristics (Signal-to-Noise-Ratio(SNR)) and Amount of Reproduction Output During Repeated High-SpeedRunning in Environment of High Temperature and High Humidity

Regarding each magnetic tape of the examples and the comparativeexamples, the electromagnetic conversion characteristics (SNR) and theamount of a decrease in reproduction output during the repeated runningwere measured by the following method by using a reel tester having awidth of ½ inches (0.0127 meters) and including a fixed head. Themeasurement was performed in an environment of an atmosphere temperatureof 32° C. and relative humidity of 80%.

A head/tape relative speed was set as 8.0 m/sec, a metal-in-gap (MIG)head (gap length of 0.15 μm, track width of 1.0 μm) was used as therecording head, and a recording current was set as an optimal recordingcurrent of each tape. As a reproducing head, a giant-magnetoresistive(GMR) head having an element thickness of 15 nm, a shield interval 0.1μm, and a lead width of 0.5 μm was used. A signal having linearrecording density of 300 kfci was recorded and measurement regarding areproduction signal was performed with a spectrum analyzer manufacturedby Shibasoku Co., Ltd. The unit, kfci, is a unit of linear recordingdensity (not able to convert to the SI unit system). Regarding thesignal, a signal which was sufficiently stabilized after starting therunning of the magnetic tape was used. Under the conditions describedabove, the sliding of 500 passes was performed by sliding 1,000 m per 1pass to perform the recording and reproducing. A ratio of an outputvalue of a carrier signal and integrated noise of the entire spectralrange was set as an SNR. Regarding the SNR of the first pass,Broadband-SNR (BB-SNR) was shown in Table 1 as a relative value in acase where the value of Comparative Example was set as a reference (0dB).

An output value of a carrier signal of the first pass and an outputvalue of a carrier signal of 500-th pass were respectively obtained, anda difference of “(output value of 500-th pass)−(output value of firstpass)” was shown in Table 1 as the amount of a decrease in reproductionoutput during the repeated running.

Regarding the magnetic tape of Comparative Example 7, it was difficultto cause the magnetic tape to run due to sticking between the surface ofthe magnetic layer and the head, and thus, the evaluation was stopped.

(2) Evaluation of Amount of Head Attached Materials

After the measurement in the section (2), the surface of the reproducinghead after reciprocating of 500 passes was observed with a differentialinterference microscope, and the amount of head attached materials wasdetermined with the following criteria, in accordance with the size ofthe area in which the attached materials were confirmed in a microscopicimage obtained with the differential interference microscopeobservation.

5 points: Substantially no head attached materials were observed.

4 points: a slight amount of head attached materials was observed.

3 points: Head attached materials were observed (the amount thereof isgreater than that in a case of 4 points and smaller than that in a caseof 2 points).

2 points: A large amount of head attached materials was observed.

1 point: An extremely large amount of head attached materials wasobserved.

The results described above are shown in Table 1.

TABLE 1 Comparative Comparative Comparative Comparative ComparativeComparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6Colloidal silica average particle size 120 80 80 80 80 80 (nm) Calendertemperature (° C.) 110 110 110 110 110 110 Cooling zone staying time NotNot 1 second 60 seconds 180 seconds 180 seconds performed performedBurnishing treatment before curing Not Not Performed Performed PerformedPerformed process performed performed Burnishing treatment after curingPerformed Performed Not Not Not Not process performed performedperformed performed Non-magnetic layer forming 4.0 4.0 4.0 4.0 4.0 4.0composition Amount of butyl stearate (part) Ultrasonic Vibrationimparting time None None None None None 0.5 vibration Frequency NoneNone None None None 30 imparting Intensity None None None None None 100conditions Magnetic Flow-type ultrasonic 2 times 2 times 2 times 2 times2 times 1 time layer dispersing device forming Number of times of passescomposition Number of times of 1 time 1 time 1 time 1 time 1 time 1 timepreparation filtering conditions Filter hole diameter 1.0 μm 1.0 μm 1.0μm 1.0 μm 1.0 μm 1.0 μm Magnetic Center line average 2.2 1.7 1.7 1.7 1.71.7 layer surface roughness Ra (nm) Logarithmic decrement 0.062 0.0600.048 0.030 0.015 0.015 S_(after)-S_(before) (nm) 4.0 4.0 3.9 4.0 4.03.9 FWHM_(before) (nm) 8.0 7.9 8.0 8.0 7.9 6.6 FWHM_(after) (nm) 6.7 6.66.6 6.7 6.7 8.0 BB-SNR/dB 0 3.2 3.3 3.2 3.3 3.2 Head attached materials(large: 1 ⇔ 5 1 2 3 4 4 5: small) Decrease in reproduction output (dB)−0.8 −4.0 −2.6 −2.1 −1.9 −2.0 Comparative Comparative ComparativeExample 7 Example 8 Example 9 Example 1 Example 2 Example 3 Colloidalsilica average particle size 80 80 80 80 80 80 (nm) Calender temperature(° C.) 110 110 110 110 110 110 Cooling zone staying time 180 seconds 180seconds Not 1 second 5 seconds 60 performed seconds Burnishing treatmentbefore curing Performed Performed Not Performed Performed Performedprocess performed Burnishing treatment after curing Not Not PerformedNot Not Not process performed performed performed performed performedNon-magnetic layer forming 10.0 0.0 4.0 4.0 4.0 4.0 composition Amountof butyl stearate (part) Ultrasonic Vibration imparting time 0.5 0.5 0.50.5 0.5 0.5 vibration Frequency 30 30 30 30 30 30 imparting Intensity100 100 100 100 100 100 conditions Magnetic Flow-type ultrasonic 2 times2 times 2 times 2 times 2 times 2 times layer dispersing device formingNumber of times of passes composition Number of times of 1 time 1 time 1time 1 time 1 time 1 time preparation filtering conditions Filter holediameter 1.0 μm 1.0 μm 1.0 μm 1.0 μm 1.0 μm 1.0 μm Magnetic Center lineaverage 1.7 1.7 1.7 1.7 1.7 1.7 layer surface roughness Ra (nm)Logarithmic decrement 0.015 0.015 0.062 0.048 0.040 0.032S_(after)-S_(before) (nm) 8.2 0 4.0 4.0 4.0 4.0 FWHM_(before) (nm) 6.76.6 6.7 6.7 6.6 6.6 FWHM_(after) (nm) 6.6 6.7 6.6 6.7 6.6 6.6 BB-SNR/dB3.2 3.2 3.2 3.3 3.2 3.2 Head attached materials (large: 1 ⇔ — 1 4 5 5 55: small) Decrease in reproduction output (dB) — −3.0 −2.0 −0.8 −0.6−0.4 Example 4 Example 5 Example 6 Example 7 Example 8 Colloidal silicaaverage particle size 80 80 80 80 40 (nm) Calender temperature (° C.)110 110 110 110 90 Cooling zone staying time 180 1 second 1 second 1second 60 seconds seconds Burnishing treatment before curing PerformedPerformed Performed Performed Performed process Burnishing treatmentafter curing Not Not Not Not Not process performed performed performedperformed performed Non-magnetic layer forming 4.0 8.0 4.0 4.0 4.0composition Amount of butyl stearate (part) Ultrasonic Vibrationimparting time 0.5 0.5 3 0.5 3 vibration Frequency 30 30 30 30 30imparting Intensity 100 100 100 100 100 conditions Magnetic Flow-typeultrasonic 2 times 2 times 2 times 30 times 30 times layer dispersingdevice forming Number of times of passes composition Number of times of1 time 1 time 1 time 5 times 5 times preparation filtering conditionsFilter hole diameter 1.0 μm 1.0 μm 1.0 μm 0.5 μm 0.5 μm Magnetic Centerline average 1.7 1.7 1.7 1.7 1.5 layer surface roughness Ra (nm)Logarithmic decrement 0.014 0.048 0.048 0.047 0.030 S_(after)-S_(before)(nm) 4.1 7.0 4.0 3.9 4.0 FWHM_(before) (nm) 6.6 6.7 4.0 6.7 3.9FWHM_(after) (nm) 6.7 6.6 6.6 4.0 4.0 BB-SNR/dB 3.3 3.2 3.2 3.2 4.1 Headattached materials (large: 1 ⇔ 5 5 5 5 5 5: small) Decrease inreproduction output (dB) −0.3 −0.8 −0.6 −0.5 −0.4

From results shown in Table 1, it is possible to confirm that themagnetic tapes of Examples show a high SNR and in which a decrease inreproduction output during repeated high-speed running under anenvironment of a high temperature and high humidity is small.

The invention is effective in technical fields of magnetic tapes used asrecording media for data storage.

What is claimed is:
 1. A magnetic tape comprising: a non-magneticsupport; and a magnetic layer including ferromagnetic powder and abinding agent on the non-magnetic support, wherein the center lineaverage surface roughness Ra measured regarding the surface of themagnetic layer is equal to or smaller than 1.8 nm, the logarithmicdecrement acquired by a pendulum viscoelasticity test performedregarding the surface of the magnetic layer is 0.014 to 0.050, themagnetic layer includes fatty acid ester, the full width at half maximumof spacing distribution measured by optical interferometry regarding thesurface of the magnetic layer before performing vacuum heating withrespect to the magnetic tape is greater than 0 nm and equal to orsmaller than 7.0 nm, the full width at half maximum of spacingdistribution measured by optical interferometry regarding the surface ofthe magnetic layer after performing the vacuum heating with respect tothe magnetic tape is greater than 0 nm and equal to or smaller than 7.0nm, the difference S_(after)−S_(before) between a spacing S_(after)measured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape and a spacing S_(before) measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is greater than 0 nm and equalto or smaller than 8.0 nm, and the logarithmic decrement on the magneticlayer side is determined by the following method: securing a measurementsample of the magnetic tape with the measurement surface, which is thesurface on the magnetic layer side, facing upward on a substrate in apendulum viscoelasticity tester; disposing a columnar cylinder edgewhich is 4 mm in diameter and equipped with a pendulum 13 g in weight onthe measurement surface of the measurement sample such that the longaxis direction of the columnar cylinder edge runs parallel to thelongitudinal direction of the measurement sample; raising the surfacetemperature of the substrate on which the measurement sample has beenpositioned at a rate of less than or equal to 5° C./min up to 80° C.;inducing initial oscillation of the pendulum; monitoring thedisplacement of the pendulum while it is oscillating to obtain adisplacement-time curve for a measurement interval of greater than orequal to 10 minutes; and obtaining the logarithmic decrement A from thefollowing equation:$\Delta = \frac{{\ln\left( \frac{A_{1}}{A_{2}} \right)} + {\ln\left( \frac{A_{2}}{A_{3}} \right)} + {\ldots\mspace{14mu}{\ln\left( \frac{A_{n}}{A_{n + 1}} \right)}}}{n}$wherein the interval from one minimum displacement to the next minimumdisplacement is adopted as one wave period; the number of wavescontained in the displacement-time curve during one measurement intervalis denoted by n, the difference between the minimum displacement and themaximum displacement of the n^(th) wave is denoted by An, and thelogarithmic decrement is calculated using the difference between thenext minimum displacement and maximum displacement of the n^(th) wave(A_(n+1) in the above equation).
 2. The magnetic tape according to claim1, wherein the center line average surface roughness Ra is 1.2 nm to 1.8nm.
 3. The magnetic tape according to claim 1, wherein the full width athalf maximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer before performing the vacuumheating with respect to the magnetic tape is 3.0 nm to 7.0 nm.
 4. Themagnetic tape according to claim 1, wherein the full width at halfmaximum of spacing distribution measured by optical interferometryregarding the surface of the magnetic layer after performing the vacuumheating with respect to the magnetic tape is 3.0 nm to 7.0 nm.
 5. Themagnetic tape according to claim 1, wherein the differenceS_(after)−S_(before) is 2.0 nm to 8.0 nm.
 6. The magnetic tape accordingto claim 1, wherein the logarithmic decrement is 0.014 to 0.050, thecenter line average surface roughness Ra is 1.2 nm to 1.8 nm, the fullwidth at half maximum of spacing distribution measured by opticalinterferometry regarding the surface of the magnetic layer beforeperforming the vacuum heating with respect to the magnetic tape is 3.0nm to 7.0 nm, the full width at half maximum of spacing distributionmeasured by optical interferometry regarding the surface of the magneticlayer after performing the vacuum heating with respect to the magnetictape is 3.0 nm to 7.0 nm, and the difference S_(after)−S_(before) is 2.0nm to 8.0 nm.
 7. The magnetic tape according to claim 1, furthercomprising: a non-magnetic layer including non-magnetic powder and abinding agent between the non-magnetic support and the magnetic layer.8. The magnetic tape according to claim 1, further comprising: a backcoating layer including non-magnetic powder and a binding agent on asurface side of the non-magnetic support opposite to a surface sideprovided with the magnetic layer.